Among the non-volatile semiconductor memory devices represented as an electrically re-writable EEPROM (Electrically Erasable and Programmable ROM), some of them have a laminated structure such as SONOS (Silicon-Oxide-Nitride-Oxide-Silicon) type or MONOS (Metal-Oxide-Nitride-Oxide-Silicon) type. In these types of non-volatile semiconductor memory devices, information is retained by one or more layers of silicon nitride film, formed between silicon dioxide films (oxide), as a charge-accumulating area. That is, in the non-volatile semiconductor memory devices a voltage is applied between a semiconductor substrate (silicon) and a control gate electrode (silicon or metal) to inject electrons into the silicon nitride film of the charge-accumulating area thereby storing data. Or, data may be re-written for storing or erasing by eliminating electrons accumulated in the silicon nitride film. It is believed that a charge-accumulating capacity of the silicon nitride films is relevant to the band gap structure thereof.
As for the technology for forming a silicon nitride films working as a charge-accumulating area of a non-volatile semiconductor memory device, Patent Literature 1 discloses a method for forming a silicon nitride film between a tunnel oxide film and a top oxide film by a depressurized CVD (Chemical Vapor Deposition) method under the condition that raw gas is dichlorosilane (SiH2Cl2) and ammonia (NH3), and the flow ratio of SiH2Cl2/NH3 is equal to or less than 1/10. However, it is difficult to control the band gap of the respective silicon nitride film only with the process conditions in the film-producing process by the conventional CVD method. In the conventional process, so as to control the band gap size in the silicon nitride film, after forming the silicon nitride film by the CVD method, the components of the film itself need to be changed by, for example, oxidizing the silicon nitride film to make a silicon nitride oxide film. Therefore, so as to change the quality of the silicon nitride film by an oxidization process, a plurality of film forming devices are needed and, as a result, the processing efficiency is decreased. In particular, the processes become more complicated and the processing efficiency is further decreased when the silicon nitride film which functions as a charge-accumulating area is formed as two or more laminating structure (silicon nitride film laminate).
Though the silicon nitride films are usually formed by the plasma CVD method, most of the silicon nitride films produced by the method shows a good quality with dense and less defects to be used as a hard mask or a stopper film in an etching process.    Patent Literature 1: Japanese Unexamined Patent Publication No. Hei 5-145078 (e.g. paragraph [0015] etc.).